1. Field of the Invention
The present invention relates to a controller for an industrial machine such as a machine tool or a robot, and specifically to a receiving circuit to which position information is input from a position detector.
2. Description of the Related Art
In a controller for an industrial machine such as a numerical control device or a robot controller, position control is performed by feeding back position information and comparing it with position command information. In such position control, the position information obtained from a position detector provided at driving means such as a motor is received by a receiving circuit and signal processing is performed by a CPU provided in the controller.
FIG. 7 is a schematic block diagram showing a control system of a numerical controller. In FIG. 7, reference numeral 1a denotes a host processor for controlling the numerical controller. The host processor 1a performs interpolation, transformation of coordinate values into a rotation angle for each axis, inverse transformation, and distribution of a position command to each axis. Reference numeral 1b is a common RAM which is an intermediary through which information is transmitted between the host processor 1a and processors in axis control circuits 2x, 2y, 2z which are each formed by a digital servo circuit or the like. The common RAM 1b transmits data such as a position command or the like written by the host processor 1a to the processors in the axis control circuits 2x, 2y, 2z, and transmits alarm information or the like written by any of the processors in the axis control circuits 2x, 2y, 2z to the host processor 1a. The axis control circuits 2x, 2y, 2z are digital servo circuits which are each composed of a processor, a RAM, a ROM and the like, and perform axis position control of servomotors provided for individual axes. Reference numerals 7x, 7y, 7z denote servo amplifiers which can each be formed by a transistor inverter or the like. Sign M denotes a servomotor. Position and speed of each servomotor M can be detected by each detector 6. Detected position information is fed back to the axis control circuits 2x, 2y, 2z.
In a conventional numerical controller, for each axis, position information fed back from the detector is received by the receiving circuit 4x, 4y or 4z provided in the numerical controller 10, and fed back to the axis control circuit 2x, 2y or 2z. Based on this position information and a position command coming from the host processor 1a, position deviation is obtained, and a control signal based on this position deviation is provided to the servo amplifier 7 in order to perform position control.
In the conventional numerical controller, for each axis, position information coming from the detector is received by a single receiving circuit. The problem with this case is that if an abnormality happens to the receiving means including the, receiving circuit, it becomes difficult to receive position information from the detector.
FIG. 8 is a diagram for explaining feedback of position information in the conventional numerical controller. Here, illustration is made only for a single axis. In FIG. 8, the numerical controller 10 receives position information from the detector 6 by a single receiving circuit 4 and feeds back the position information to the axis control circuit 2. In this arrangement, if an abnormality happens to the receiving circuit 4, correct position information is not fed back, so that position control is not performed normally. For example, if position information is not output from the receiving circuit 4, position control by the axis control circuit 2 is difficult to perform.